Major US carriers don't support dual SIM at launch of iOS 12.1

Even though iPhone now supports dual SIM functionality, all three major U.S. carriers — Verizon, T-Mobile, AT&T — currently do not. Apple's technology relies on a primary physical SIM card and a virtual eSIM, each of which can be set up as a separate line.

Verizon, the largest of the three, seems to have the biggest issues lying ahead, reports PCMag. Citing people familiar with the matter, the publication lists several limitations to Apple's solution.

Currently, when a Verizon account is added to the secondary SIM, it is downgraded to 2G CDMA functionality that lacks the ability to support MMS, let alone FaceTime or any other data-intensive tasks. Verizon also says that 30 percent of their cell sites are LTE-only, which means users would experience a significant coverage hit if support rolls out as is.

These drawbacks will be present until a fix can be implemented, something Verizon says it is working on with Apple.

"Based on our discussions with Apple, we believe these concerns will be resolved quickly and you should be able to add our great Verizon service on your secondary line before the end of the year," Verizon said in a statement. "Until the secondary line can deliver Verizon's full suite of voice and high-speed data services, we won't activate Verizon service on any eSIM. This includes our own customers' iPhones with dual SIM capabilities as well as iPhones on competitors' networks."

Once the issues are mitigated, Verizon will add support for dual SIM functionality to the iPhone for two simultaneous Verizon lines.

T-Mobile, too, says it is currently working on supporting the feature and will soon enable it once the software is ready. AT&T's official position on dual SIM is unclear, but MacRumors, citing a local store manager, reports the feature is not supported at this time.

Apple in September revealed its latest iPhone XS and XR models support dual SIM functionality, allowing owners to utilize two cellular plans on a single device without constantly swapping SIM cards. The solution is ideal for roaming out of the country or setting up work and personal numbers to both go to the same phone.

A common smartphone feature in Asian countries like China, dual SIM is less popular in the U.S., but its inclusion in the latest iPhones could boost adoption.

Dual SIM was not available when iPhone XS or XR launched in September and October, respectively, but was instead implemented in the newly released iOS 12.1 update.